Solid fuel boilers are commonly employed by industrial and commercial users to generate steam, thereby reducing dependence on traditional fossil fuels as sources of energy. Such boilers typically burn solid fuels and biomass, such as bark, coal, sludge, wood waste, refuse, tire derived fuel (TDF), and other organic materials, often in combination and with the addition of fossil fuels through a process called gasification, in which energy is extracted from the solid fuels.
Typical solid fuel boilers are constructed as large boxes (up to 100 m2 or more in floor area) comprising heavy steel tubing for the walls. The tubes typically have an outside diameter of 63.5 mm or 76.2 mm and are arrayed parallel to one another, with their lengthwise ends running vertically, and spaced apart about 10-25 mm with a steel membrane or fin bridging the gaps to form substantially flat panels as walls. The entire assembly is seal welded together, forming an air tight structure. The boiler walls, or tube panels, run vertically to the top of the boiler, which can be up to 30 m or more tall. The walls are fed re-circulating water at their lower extremities by headers. Typically, the tubes forming the front wall of the box bend at the upper portion of the box to form a substantially horizontal roof over the box. The side walls culminate at the upper portion of the box in relieving headers, which in turn feed back to a steam drum. The rear wall at its upper portion either ends in a header or feeds directly into the steam drum. In order to feed fuel and combustion air into the boiler, and for other purposes, the boiler tubes are bent apart to form openings in the tube panel. There are typically multiple fuel chutes penetrating a wall or walls of the boiler. In common practice, solid fuel is gravity fed from a hopper and/or conveyor system through the large (about 0.25 m2 in area), steeply mounted chutes to the lower portion of the boiler just above the grate or fluidized bed. A solid fuel distributor is often integrally connected with the bottom portion of a chute where the chute interfaces with the boiler wall. In grate-type boilers, mechanically or pneumatically operated fuel distributors are typically required, whereas fluidized bed boilers can be operated without as the fluidized sand bed by design distributes the fuel.
Although it should be appreciated that solid fuel boilers may come in different shapes or sizes, they are distinguished primarily upon the design of their lower furnaces. To this end, solid fuel boilers are broadly categorized into either sand-bed (fluidized) or grate-fired boilers. Both types show inherent design flaws that prevent them from operating at their full capacities and/or cause them to break down frequently. Additionally, they both suffer from poor combustion efficiencies due to relatively low heat tolerance and poor control of combustion air in the lower furnaces. In either case, decreased efficiency and increased operational or maintenance costs are observed. Although the shortcomings of each type of boiler will be discussed in greater detail below, sand-bed, or fluidized-bed, boilers generally suffer from sand erosion of the parts through which high pressure water and steam are carried causing them to be de-rated or to require frequent repair. Grate-fired boilers generally suffer from high maintenance costs and operational problems associated with the moving parts comprising the grates. Both types of boiler suffer from poor combustion efficiencies due to the relatively low heat tolerance of the grate or bed and poor control of combustion air in the lower furnace.
Grate-type boilers include those with traveling grates, vibrating grates, tilting grates, or hydro-grates. In a typical grate-type boiler, the grates cover the bottom of the boiler floor and are made of heavy cast iron components with holes or slots for combustion air (called under-grate air) to be forced through from a plenum below. In operation, solid fuel lands on the grates from above and burns on the grates' upper surfaces. The resulting ash is dumped off as the grates move (rotate like a tank tread), vibrate, or tilt (in sections). Grate-type systems suffer from costly maintenance and operational problems. For instance, in the case of traveling grates, the grate is made up of hundreds of individual segments similar to chain links that form a rotating “tank tread” across the width of the boiler. These parts are subject to mechanical wear due to frictional contact between the many moving parts and attack from the hot boiler environment. Maintenance of traveling grates can typically cost tens of thousands of dollars per year, and replacement costs can amount to hundreds of thousands.
Another type of grate is the reciprocating stepped grate as described, for example, in U.S. Pat. No. 5,069,146 to Dethier, U.S. Pat. No. 4,676,176 to Bonomelli, and U.S. Pat. No. to 4,884,516 to Linsén. In the reciprocating stepped grate, reciprocating steps between fixed steps force the fuel down a series of steps. Combustion is provided between the fixed and reciprocating steps.
Operationally, grate systems suffer to some extent from problems of fuel piling and combustion air “short-circuiting.” For instance, when solid fuel lands on the grate, especially at higher load rates and/or with higher humidity content of the solid fuel, piles of fuel are often formed thereon. The piles of fuel may form with such depth and density that the grate air cannot be forced through the grate from below. Therefore, the grate air is said to “short-circuit” as it is forced around the pile, resulting in less available air as required to burn the pile and more air to burn any thinner material surrounding the pile. This scenario of short-circuiting not only exacerbates the situation of pile formation, but further results in non-uniform combustion across the hearth of the furnace. To combat this, grate-fired boilers are often run at reduced load rates, higher travel speeds, and with extra under grate air. The use of extra air, in particular, reduces the combustion and thermal efficiencies of the boiler and can lead to excess emissions. Further, moving grate systems suffer from seal failures between the grates and the boiler walls, leading to excessive air leakage and even more short-circuiting and use of excessive under grate air. Mechanical grates also must be cooled to prevent premature failure. Many grate systems rely on a large flow of under grate air for cooling. This limits the combustion control flexibility as the grate air has a large minimum air flow requirement for cooling. Hydro grates utilize water cooled tubes to support the fuel during combustion and may not require as much under grate air, however, the water cooled tubes cools the fuel pile and reduces the combustion efficiency. Due to the relatively low temperature tolerance of mechanical grates, and the inherently cool nature of hydrogrates, both of these systems must be run at temperatures much lower than optimum for combustion purposes.
Fluidized bed boilers, including those with circulating fluidized beds or other arrangements, generally have a mass of sand or other media, forming a bed across the floor of the boiler through which a stream of combustion air, or an air and boiler flue gas mixture, is percolated to fluidize the bed. In other words, due to the percolating air the sand bed behaves as a fluid, and is said to be “fluidized”. Solid fuel particles float inside the fluidized sand bed, suspended by the turbulent motion of the sand and air. The fluidized bed—comprising a hot mass of fluidized sand—acts as a heat sink, fuel drying system, turbulent fuel/air mixing system, fuel distribution system, and means for separating fuel and ash in the boiler. These boilers commonly suffer from maintenance problems because the sand is very abrasive, frequently causing leaks in the pressurized parts of the boiler. To remedy this problem, these boilers are commonly de-rated, that is, operated at less-than-optimal output. Fluidized bed boilers also suffer from poor combustion and thermal efficiencies because the amount of fluidizing air required is often dictated by the need to fluidize the bed. It is difficult, therefore, to control the amount of fluidizing air on a stoichiometric basis.
In order for the fuel to burn efficiently it must be mixed with the combustion air in an aggressive manner. Typically the fuel slides down the chute and enters the boiler with high residual moisture content (up to 50% or more). The water in the fuel inhibits combustion in the furnace, often requiring the continual use of supplemental fossil fuels or a fluidized bed to provide additional heat transfer to compensate for moisture swings. It is also very common for the load rate on these boilers to change frequently in reaction to changing steam demands. Inconsistent and high moisture content of the fuels makes it difficult for the boiler to respond effectively to the required load changes. This requires, again, the use of supplemental fossil fuels to improve the response of the boiler to load rate changes. Fossil fuels are typically used to start these boilers but continual use of fossil fuels is extremely expensive. Fluidized beds in particular can help to compensate for varying moisture contents and load rates because they act as heat sinks, but they can have significant operational and mechanical problems such as sand sintering and sand erosion and they require a sand reclamation system. Fluidized sand beds also have a temperature limit that is well below the optimum temperature of combustion for many fuels. This limits the efficiency of combustion.
To alleviate the problem of incomplete combustion, additional combustion air, typically called over fired air (OFA), is injected into these boilers above the grate or fluidized bed to help complete the combustion. In practice, however, the design of the OFA systems is seldom adequate to overcome the deficiencies of the under grate or fluidizing air system.
Embodiments of the present invention address many the aforementioned disadvantages of these types of solid fuel boilers.